A man holds an iPhone as he visits Sina's Weibo microblogging site in Shanghai May 29, 2012.

Reuters/Carlos Barria/Files

TIANJIN, China In a modern office building on the outskirts of the Chinese city of Tianjin, rows of censors stare at computer screens. Their mission: delete any post on Sina Weibo, China's version of Twitter, deemed offensive or politically unacceptable.

But the people behind the censorship of China's most popular microblogging site are not ageing Communist Party apparatchiks. Instead, they are new college graduates. Ambivalent about deleting posts, they grumble loudly about the workload and pay.

Managing the Internet is a major challenge for China. The ruling Communist Party sees censorship as key to maintaining its grip on power - indeed, new measures unveiled on Monday threaten jail time for spreading rumours online.

At the same time, China wants to give people a way to blow off steam when other forms of political protest are restricted.

Reuters interviewed four former censors at Sina Weibo, who all quit at various times this year. All declined to be identified because of the sensitivity of the work they once did. Current censors declined to speak to Reuters.

"People are often torn when they start, but later they go numb and just do the job," said one former censor, who left because he felt the career prospects were poor. "One thing I can tell you is that we are worked very hard and paid very little."

Sina Corp (SINA.O), one of China's biggest Internet firms, runs the microblogging site, which has 500 million registered users. It also employs the censors.

The company did not respond to repeated requests for comment.

"STRESSFUL, DEAD-END JOB"

Reuters got a glimpse of the Sina Weibo censorship office in Tianjin, half an hour from Beijing by high-speed train, one recent weekend morning.

A dozen employees, all men, could be seen through locked glass doors from a publicly accessible corridor, sitting in cramped cubicles separated by yellow dividers, staring at large monitors.

They more closely resembled Little Brothers than the Orwellian image of an omniscient and fearsome Big Brother.

"Our job prevents Weibo from being shut down and that gives people a big platform to speak from. It's not an ideally free one, but it still lets people vent," said a second former censor.

The former censors said the office was staffed 24 hours a day by about 150 male college graduates in total. They said women shunned the work because of the night shifts and constant exposure to offensive material.

The Sina Weibo censors are a small part of the tens of thousands of censors employed in China to control content in traditional media and on the Internet.

Most Sina Weibo censors are in their 20s and earn about 3,000 yuan a month, the former censors said, roughly the same as jobs posted in Tianjin for carpenters or staff in real estate firms. Many took the job after graduating from local universities.

"People leave because it's a stressful dead-end job for most of us," said a third former censor.

Sina's computer system scans each microblog before they are published. Only a fraction are marked as sensitive and need to be read by a censor, who will decide whether to spare or delete it. Over an average 24-hour period, censors process about 3 million posts.

A small number of posts with so-called "must kill" words such as references to the banned spiritual group Falun Gong are first blocked and then manually deleted. Censors also have to update lists of sensitive words with new references and creative expressions bloggers use to evade scrutiny.

For most posts deemed sensitive, censors often use a subtle tactic in which a published comment remains visible to its author but is blocked for others, leaving the blogger unaware his post has effectively been taken down, the former censors said. Censors can also punish users by temporarily blocking their ability to make comments or shutting their accounts in extreme cases.

"We saw a fairly sophisticated system, where human power is amplified by computer automation, that is capable of removing sensitive posts within minutes," said Jedidiah Crandall of the University of New Mexico, part of a team which did recent research on the speed of Weibo censorship.

If a sensitive post gets missed and spreads widely, government agencies can put pressure on Sina Corp to remove the post and occasionally punish the censor responsible with fines or dismissal, the former censors said.

On an average day, about 40 censors work 12-hour shifts. Each worker must sift through at least 3,000 posts an hour, the former censors said.

The busiest times are during sensitive anniversaries such as the Tiananmen Square crackdown on pro-democracy protesters which took place on June 4, 1989, and major political events.

The censors shifted into high gear during the downfall last year of former high-flying politician Bo Xilai, who faced trial last month on charges of bribery, graft and abuse of power. A verdict may come this month.

"It was really stressful, about 100 people worked non-stop for 24 hours," the first censor said, referring to when Bo was stripped of his posts and later expelled from the Party.

"FREEDOM MEANS ORDER"

The Communist Party keeps an iron grip on newspapers and television but has grappled to control information on social-networking platforms.

Internet firms are required to work with the party's propaganda apparatus to censor user-generated content.

Lu Wei, director of the State Internet Information Office, said in a speech this week that "freedom means order" and that "freedom without order does not exist".

State media has reported dozens of detentions in recent weeks as the new government of President Xi Jinping cracks down on the spreading of rumours.

China's top court and prosecutor said people would be charged with defamation if online rumours they created were visited by 5,000 Internet users or reposted more than 500 times.

That could lead to three years in jail, state media reported on Monday. China says it has a genuine need to stop the spread of irresponsible rumours.

When rumours that former president Jiang Zemin had died went viral on Weibo, the seemingly irrelevant words "frog" and "toad", most likely referring to Jiang's peculiar glasses, were used to refer to Jiang and later banned.

Censors are told what kinds of comments are off limits.

"The most frequently deleted posts are the political ones, especially those criticising the government, but Sina grants relatively more room for discussions on democracy and constitutionalism because there are leaders who want to keep the debate going," said the first former censor.

"But there hasn't been any sign of loosening control on social media since Xi Jinping took power," he added. "Not from what we could feel at work."